Application Note08-2016
Measuring Salinity
of Water
Measuring the salinity or the
dissolved salt content of water is
important as aquatic organisms,
livestock, and crops thrive at
different salinity levels. Freshwater
has a salinity value of less than
0.5 ppt while seawater has an
average salinity of 35 ppt.
Introduction
Salinity is the measure of the amount of dissolved salts in water. It is
usually expressed in parts per thousand (ppt) or percentage (%).
Freshwater from rivers has a salinity value of 0.5ppt or less. Within
the estuary, salinity levels are referred to as oligohaline (0.5-5.0 ppt),
mesohaline (5.0-18.0 ppt), or polyhaline (18.0-30.0 ppt). Near the
connection with the open sea, estuarine waters may be euhaline, where
salinity levels are the same as the ocean at more than 30.0 ppt.1
Salinity varies from place to place in the oceans, but the relative
proportions of the most major dissolved constituents remain virtually
constant. Even though there are smaller quantities of other ions in
seawater (e.g., K+, Mg2+, SO42-), sodium (Na+) and chloride (Cl-) ions
represent about 91% of all the seawater ions. Freshwater has much
lower levels of salt ions.2
Salinity is often derived from electrical conductivity (EC) measurement.
EC is measured by passing an electric current between two metal plates
or electrodes in the water sample and measuring how readily current
flows between the plates. The use of EC measurements to estimate the
ionic content of seawater led to the development of Practical Salinity
Scale 1978 (PSS-78).3
The PSS-78 has been considered by the Joint Panel on Oceanographic
Tables and Standards and recommended by all oceanographic
organizations as the scale in which to report future salinity data.
The practical salinity of a sample of seawater is defined in terms of
the ratio of the electrical conductivity of the seawater sample at the
temperature of 15ºC and the pressure of 1 standard atmosphere,
to that of a potassium chloride (KCl) solution containing a mass of
32.4356 g KCl in a mass of 1 kg of solution at the same temperature
and pressure. A ratio equal to 1 corresponds to a practical salinity 35
(standard seawater).4 As the definition is a ratio, practical salinity is
expressed as dimensionless number.
The LAQUAtwin Salt 11 pocket meter measures the conductivity value
of a sample then converts it to salinity value based on the salinity
standard curve selected. The sensor has two titanium metals coated
with platinum black that resist corrosion and temperature sensor for
accurate measurement. The meter is programmed with two standard
calibration curves—seawater and sodium chloride (NaCl). The former
Non-Tidal
Fresh Water
Average
Annual
Salinity
Tidal
Fresh Water
Tidal Limit
(During
Low Flow
Conditions)
<0.5 ppt
<5.0 ppt
Mesohaline
<18.0 ppt
Polyhaline
<30.0 ppt
Euhaline
OCEAN
(Marine)
Figure 1: Estuarine salinity slowly increases as one moves away from freshwater
sources and toward the ocean.
Source: Chapter 14 Salinity. Voluntary Estuary Monitoring Manual.
Continued at the back
Continued from the front
follows the PSS-78 equation while the latter
follows the equation in Figure 2.
300
COND / mS/cm-1
250
200
100
50
0
1000
2000
3000
4000
5000
Salinity is an important measurement in
seawater or in estuaries where freshwater
from rivers or streams mixes with salty
ocean water since aquatic organisms
have varying abilities to survive and
thrive at different salinity levels. Saltwater
organisms survive in salinity levels up to
40 ppt, yet many freshwater organisms
cannot live in salinity levels above 1 ppt.2
Before testing another sample, rinse the
sensor with DI (distilled or deionized)
water or with the next sample to be
tested and dab a soft tissue to remove
remaining water trapped inside the
sample well.
150
0
Results and Benefits
using the pipette included in the kit.
Make sure that sample well is completely
filled-up with sample and that there are
no bubbles formed. Record the salinity
reading once it is stable.
6000
NaCl / mM
Figure 2: NaCl Curve
Salinity affects the dissolved oxygen levels
in water. The solubility of oxygen in water
decreases as salinity increases. The
solubility of oxygen in seawater is about
20% less than it is in fresh water at the
same temperature.5
Refer to Technical Tip 3 for detailed
information on conditioning, cleaning,
and storing the salt sensor. The technical
tip can be viewed and downloaded
from the support section of our website
www.horiba-laqua.com.
Method
Calibrate the LAQUAtwin Salt 11 pocket
meter according to manufacturer’s
instructions using the 0.5% (5ppt) and
5.0% (50ppt) NaCl standard solutions
included in the kit.
The table below shows the salinity values
of different types of water and their uses.
Table 1: Water Salinity
Salinity Status
Prior to calibration, make sure to
select the standard calibration curve
(either NaCl or Seawater) and the
unit (either ppt or %) depending on
your application and reporting unit
requirement. If both seawater curve
and ppt unit are selected, calibrate the
meter with 5ppt NaCl standard. It is
best to use 5ppt and 50ppt seawater
standards, if available. Note that if ppt is
set as the measurement unit, the meter
will display only the salinity reading
without the unit (dimensionless).
Salinity (%)
Salinity (ppt)
Use
< 0.05
< 0.5
Marginal
0.05 – 0.1
0.5 – 1
Brackish
Saline
Highly Saline
Brine
0.1 – 0.2
0.2 – 1.0
1.0 – 3.5
> 3.5
1–2
2 – 10
10 – 35
> 35
Drinking and all irrigation
Most irrigation, adverse effects on ecosystems
become apparent
Irrigation certain crops only; useful for most stock
Useful for most livestock
Very saline groundwater, limited use for certain livestock
Seawater; some mining and industrial uses exist
Fresh
Source: Department of Water. Government of Western Australia.
References and Suggested Readings
1.
Chapter 14 of the Volunteer Estuary Monitoring Manual, A Methods Manual, Second Edition, EPA-842-B-06-003.
https://www.epa.gov/sites/production/files/2015-09/documents/2009_03_13_estuaries_monitor_chap14.pdf
2.
Chloride and Salinity. http://www.ruf.rice.edu/~cbensa/Salinity/
3.
Salinity. Wikipedia. https://en.wikipedia.org/wiki/Salinity
4.
Practical Salinity Scale – 1978. Salinometry. http://salinometry.com/pss-78/
5.
Salinity. Nooa Ocean Service Education. http://oceanservice.noaa.gov/education/kits/estuaries/media/supp_
estuar10c_salinity.html
6.
Understanding Salinity. Department of Water. Government of Western Australia. http://wadow.clients.squiz.net/watertopics/water-quality/managing-water-quality/understanding-salinity
Revision 1.0, 28 October 2016
Sample Collection and Measurement
Place drops of water onto the sensor
SALT 11 (NaCl) Meter
SALT 11 (NaCl)
WATER
PROOF
Features
MICRO
VOLUME
2 PT
CAL
IP67 Rated salt pocket meter with robust
titanium/platinum black cell that resists
corrosion. NaCl and seawater standard
calibration curves for accurate salinity readings
EC
CONV
Applications include
Agriculture, Shrimp Farming, Food
Quality Control, Health Management
LAQUAtwin Pocket Ion Meters Lineup
pH
COND
Na+
K+
Horiba Instruments (Singapore) Pte Ltd
83 Science Park Drive, #02-02A,
The Curie, Singapore 118258
Tel. +65 6908 9660
E-mail: [email protected]
http://www.horiba-laqua.com
NO3 –
Ca2+
IMS
Salt EC
HORIBA Group is operating Integrated Management System (IMS)
ISO9001 JOA-0298 / ISO14001 JOA-E-90039 / ISO13485
JOA-MD0010 / OHSAS18001 JOA-OH0068